Originally posted by Ikarus That's if you don't want to use the equivalent focal length on FF,
Well no, even if you use the equivalent focal length on FF. Using the equivalent focal length say 400 on APS_c and 600 on FF, using the same shutter speed, and aperture, you need to use a stop higher ISO on FF to match the DOF of APS-c.
Those things being assumed, a 24 MP 400mm1/1000s ƒ5.6 APS-c taken at 800 ISO will give you the same image as 600mm FF, taken at 1/1000s @ ƒ8 and 1600 ISO. If you keep the DoF the same, everything else is the same including noise.
The only way to get better noise performance with an FF is to sacrifice DoF. It may be a nice feature to have, but it's not without cost.
If you have a 3200 ISO image shot at ƒ4, on FF, shoot the same image @F2.8 on APS_c with the equivalent lens, and you have the same noise level. The FF advantage is when you are shooting wide open, sacrificing DoF for noise reduction. As long as you can open your APS-c system one stop and reduce your ISO, in this example from 3200 to 1600, you can get an equivalent image in APS-c.
Of course for images where you actually want narrow DoF, then FF is ahead of the game, but that is easily seen in one's photos and exif. It amazes me how many people proclaim FF superiority without even understanding all the parameters. They just repeat it like a mantra.
An FF camera is a better choice, if and only if, you need more resolution , or you shoot a lot of images less than one stop from the maximum aperture of your lenses. For everything else, equivalence rules.
I can't tell you how many times going through IR images, that the APS-c image was better than the comparable FF image, because IR shot all their images at the same ƒ-stop without regard to DoF. So the 200 ISO FF image might be better than the APS-c image in noise, but you can't see the difference or it's very minimal, but the APS-c image was better because of wider DoF, more of the test subject was in focus. By any real world measure, the APS-c image was the better image. That you will always get a better image using an FF, shooting the same image even at the same settings, is just a complete fallacy not bourn out by the study of the thousands of real world comparative images taken at Imaging Resources. Yes, you might get a better image, on average as an FF shooter, but they might be worse as well on average. It depends on how you shoot. The whole "of course every FF images has lower noise than any APS-c" is nonsense. Only if you compare apples to oranges.